This invention generally relates to current distribution devices and, more particularly, to molded commutator assemblies and a method of making same.
A conventional commutator assembly normally includes a core of insulating base material having a plurality of commutator segments disposed around the core. The commutator segments are separated from each other and extend longitudinally of the core and commutator assembly. Some form of locating or securing means, such as hooks, are provided at one end of the commutator segments for receiving and holding the coil ends of an appropriate motor armature in engagement with respective ones of the commutator segments for subsequent fusing to the segments. Small motor commutators typically are secured to armature shafts by means of a press interference fit.
One of the problems in designing and manufacturing commutator assemblies of the character described is that of providing an adequate bond between each commutator segment and the molded insulation material of the core without expensive manufacturing and assembly techniques. The bond is important to prevent the commutator segments from becoming loosened from the core which would result in an out-of-round condition wherein the outer surface of some of the segments are at a greater distance from the rotational axis than the remaining segments. This occurrence results in an increase in electrical and audible noise in the vicinity of the motor due to uneven brush contact with the commutator segments as the rotor is rotating. It also drastically reduces the life of the brushes, causing premature failure of the motor due to brush wear. An inadequate bond also could cause the commutator segments to be completely pulled off of the insulating core when the coil ends are secured to the hooks, for instance, during the assembly process by fabricating machines.
In an attempt to overcome the above problem, commutator assemblies have been designed wherein a plurality of tangs are formed in an inner surface of a tubular shell of electrically conductive material. The shell is bonded to an insulating core so that the core and the tangs are in interfering relationship. The shell then is slotted to form commutator bars or segments each having generally the same number of tangs independent of the number of bars cut from the shell. Such a commutator is disclosed in U.S. Pat. No. 2,658,159 to Herbst, dated Nov. 3, 1953 and assigned to the assignee of this invention.
Other patents disclosing commutator constructions are shown in U.S. Pat. No. 3,006,387 to C. A. Herbst, dated Dec. 4, 1962 and assigned to the assignee of this invention; and British Patent Specification No. 1,561,460 which was published Feb. 20, 1980.
Another approach to solving the problem of loosened commutator bars or segments is shown in U.S. Pat. No. 3,376,443 to McColl, dated Apr. 2, 1968. This patent shows a commutator assembly and method of making the assembly wherein a commutator shell is constructed from a flat ribbon or web of electrically conductive material, such as copper. The method of constructing the shell includes the steps of rolling one face of the web to form a plurality of longitudinal continuous dovetailed lands spaced from one another across the width of the ribbon or web, with a portion of each land overhanging an adjacent groove. The dovetailed configurations are formed by a series of steps which first form V-shaped grooves in the web and then flattening the tops of the resulting ribs between the grooves. A length of the rolled web then is severed and formed into a cylinder such that the longitudinal lands extend circumferentially on the inside of the web. A core is molded inside the resulting commutator shell so that the core material is disposed in the grooves between the dovetailed lands, and in particular so that a portion of the core material is trapped behind the overhanging portion of the lands. The shell is then slit lengthwise into a plurality of commutator segments or bars.
The McColl method has provided an improved bond between the commutator segments and the molded core because of the trapped core material behind the overhanging lands. However, the method of manufacturing the commutator shell, as described above, does not form a consistent dovetail configuration and has created severe problems in the integrity of the commutator shell and/or the commutator bars themselves. During the rolling process to form the dovetailed rib and groove pattern, considerable material, such as the copper, must be displaced. During displacement, fissures or "cracks" are formed in the commutator segments, normally at the bottom or base of the grooves that separate the dovetailed lands. As the brushes wear the outside of the commutator bars, the cracks actually can become exposed and create a current discontinuity as well as a rough surface which results in the aforesaid electrical or audible noise. These cracks contribute significantly to reducing the brush life of the motor.
In essence, it is important to provide an uninterrupted uniform thickness of electrically conductive material, such as the copper, along the area of the commutator bars. Prior attempts to provide a dovetailed rib and groove pattern have failed in this critical parameter.
This invention is directed to solving one or more of the above-identified problems.